Property of a limit of functions of average value zero in L^2 space

1. Feb 11, 2013

lmedin02

1. The problem statement, all variables and given/known data

Let $f_k\rightarrow f$ in $L^2(\Omega)$ where $|\Omega|$ is finite. If $\int_{\Omega}{f_k(x)}dx=0$ for all $k=1,2,3,\ldots$, then $\int_{\Omega}{f(x)}dx=0$.

2. Relevant equations

3. The attempt at a solution
I started by playing around with Holder's inequality and constructing examples where this is the case. Since every other example I create usually does not converge to a function. I used functions defined on a bounded symmetric interval like $\dfrac{1}{k}x$, $\sin(\dfrac{1}{k}x)$. However, I do not see how to actually set up to make this conclusion.

2. Feb 11, 2013

jbunniii

Re: Property of a limit of functions of average value zero in L^2 spac

What if you write
\begin{align} \left| \int_{\Omega} f(x) dx \right| &= \left| \int_{\Omega} f(x) dx - \int_{\Omega} f_k(x) dx \right| \\ &= \left|\int_{\Omega} (f(x) - f_k(x)) dx \right| \\ \end{align}
and apply an appropriate inequality to the right hand side?

3. Feb 11, 2013

lmedin02

Re: Property of a limit of functions of average value zero in L^2 spac

Got it. Thanks. Using Holder's inequality and noting that the size of $\Omega$ is finite the results follows since $f_k\rightarrow f$ in $L^2(\Omega)$.

Last edited: Feb 11, 2013
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